Low-voltage circuit breakers having high ratings generally utilize separable contacts arranged at the entry of an arc extinguishing chamber. When the contacts separate or open in response to a trip device following an overcurrent or the like, an electrical arc arises between the contacts. The arc extinguishing chamber is designed to absorb the energy of the arc while maintaining its voltage. Both the chamber and the separable contacts may be subject to high thermal, mechanical and electrical stresses. For example, a current of as much as 200,000 amperes may be maintained for 4 milliseconds at an arcing voltage of 500 volts, resulting in an energy of 400 kilojoules. The plasma column forming this arc can reach a temperature of as much as 4,000° to 20,000° Kelvin.
The arc extinguishing chamber includes a number of separators which are designed to break the arc down into fractions, enabling the voltage of the arc to be increased and the arc to be cooled by heat exchange with the separators. In addition to the separators, the arc chamber usually includes a filter assembly or a gas deionization device. This device may be designed as a porous shield arranged near an outlet orifice of the arc distinguishing chamber, such as a labyrinth-type of device formed by a plurality of shields with offset openings or windows.
It is important to ensure that all of the interruption gasses pass through the filters, avoiding leakage paths around the filter within the filter housing. That is, it is important to ensure that the gases within the filter housing do not pass around the actual filter elements.
Therefore, the deionization device or filter assembly and the separator assembly should be accurately assembled with each other and with the arc chamber formed in the circuit breaker housing, and held in place in proper alignment throughout the service life of the breaker, to assure that there is no significant “leakage” of arcing products around the separators and filter. Also, exposure of the arc plates or separators to heat and pressure from arc interruption can cause the plates to warp, causing a short circuit between adjacent plates, if touching. Accordingly, the assembly should provide structural integrity of the arc stack or separator assembly and filter assembly as well as sufficient support to withstand arcing forces.
In the present invention, we have also discovered a manner in which to construct the filter so as to increase the effective resistance of the filter and reduce the amount of leakage current during short circuits, so as to increase and maintain the interruption quality.